This invention relates in particular to a platform with tensioned lines for very deep water, used in particular in the petroleum industry for exploiting marine deposits. It possesses namely as a characteristic feature tensioned lines made of a material that is not very sensitive to fatigue stresses, and which are sized independently of constraints associated with periods of excitation due to the external environmental loads (swell, wind, current), and with fatigue phenomena associated with the dynamic behavior of the said platform under the effect of these loads.
The invention is applied in the field of platforms comprising anchoring lines made of a material having a high strength, for example, special high-strength steels, or tensioned lines made of high-strength carbon fiber.
Tension leg platforms, or TLPs are floating systems used for example within the context of exploiting petroleum deposits. These floating systems possess a characteristic or main original feature in that they are fitted with a tensioned anchoring system which serves to eliminate certain movements associated with swell or tides (heave, roll and pitch). Movements such as the rotation of the vertical axis (known under the term yaw by a person skilled in the art) and horizontal displacements of limited and long-period amplitude, are authorized within certain admissible limits. The anchoring system is generally made of tendons or tensioned lines, generally of a tubular shape, arranged vertically so as to hold the platform in place on the sea bed.
Another characteristic feature of the floating system is that it is always under positive tension so as to avoid compression of the lower section of the tendons under the effect of loadings resulting from the action of swell tide or other actions due to the environment. These external loadings may induce significant tension fatigue effects which may reduce the service life of the system in the long term.
If the anchor lines are made of steel, the value of the natural period of the floating system is situated within a range of values sufficiently remote from those of the periods of external loadings.
Such a floating system comprising steel tendons is particularly well-suited to relatively deep water, of the order of 1000 meters for example.
In the case of water deeper than 1000 meters, or deeper even than 1500 to 2000 meters for example, the weight of the steel anchor lines becomes an important parameter which must be taken into account in the sizing of the tension leg platform or TLP. This consideration generally leads to the TLP being oversized.
In fact, in the case of very deep water, and under the effect of the hydrostatic pressure of circumferential crushing, the own weight of the steel tendon starts to increase significantly, inducing an increase in the displacement of the floating structure which must be sufficient to support its weight. This displacement itself leads to an increase in the loads stressing the tendons, thus requiring the thickness of the steel tendons to be increased, which again implies an increase in the movement of the floating structure and so on. This sizing process is likely to lead to a divergence in the sizing of platforms for very deep seas.
To resolve this problem a prior art is known of using tendons made of light material with high-performance mechanical properties and suitable for constraints due to the environment, whilst remaining within a range of natural periods of vibration located outside the range of periods of existing external loadings or excitations.
It would be possible to use titanium. However, this has the disadvantage in that it has inappropriate longitudinal rigidity and an unsuitable density, and is also very expensive.
Composite materials enable a good compromise to be reached between mechanical strength and the cost of the tendon. Carbon fiber, for example, offers the best advantages due to its rigidity which is close to or greater than that of steel (Young""s modulus between 230 and 400 GPa, or even greater), its very low density (1.7 in air or 0.7 in water) and its very high mechanical performance (rupture strength greater than 3500 MPa accompanied by a quasi-insensitivity to fatigue and to corrosion).
This invention relates namely to a floating system for deep water comprising at least a floating structure held in place on the sea bed by means of tensioned lines, sized independently of the fatigue phenomena associated namely with the dynamic behavior of the floating structure under the effect of external loadings.
The invention relates to a floating system for deep water comprising at least a floating structure subject to external loadings (swell, wind, tide, for example) inducing stresses within the said floating system, the said floating structure being held on the sea bed by means of one or several tensioned lines made of a material having given mechanical properties.
The system is characterized in that the tensioned line or lines are made of a material which is not very sensitive to fatigue stresses and in that the tensioned line or lines are sized independently of the fatigue phenomena associated with the dynamic behavior of the floating system under the effect of external loadings. The system has several natural periods Tj, of heave T1, roll T2 or of pitch T3, and at least one of these three values (T1, T2, T3) is within the range of the periods Te of the external loadings, such as the wave excitation.
The system is characterieed in that the said tensioned line or lines are made of a material which is not very sensitive to fatigue stresses and in that the said tensioned line or lines are sized independently of the fatigue phenomena associated with the dynamic behavior of the said floating system under the effect of external loadings. The system has several natural periods Tj, of heave T1, roll T2 or of pitch T3, and at least one of these three values (T1, T2, T3) is within the range of the periods Te of the external loadings, such as the wave excitation.
The tensioned lines may be sized independently of the range of periods of excitation.
In accordance with a specific embodiment, the tensioned line or lines possess geometric characteristics such as section Si and/or diameter Di, at least one of the two characteristics being determined for example so that the stresses "sgr"i, taking into account the dynamic amplification factor FAD acting on the tensioned line or lines are less than a maximum fixed stress "sgr"max.
The tensioned line or lines may be made of high-strength carbon fiber.
In another specific embodiment the tensioned line or lines are for example made of steel cables with high mechanical strength.
At least one of the natural periods T1, T2 or T3 is for example at least greater than 7 seconds and preferably located between 7 and 12 seconds inclusive.
In accordance with a specific embodiment, the tensioned line or lines are aligned in an approximately vertical direction.
According to another specific embodiment, the tensioned line or lines form for example an angle at least equal to 10xc2x0 in relation to a vertical line and preferably between 10xc2x0 and 45xc2x0 inclusive.
The floating structure may be a marine production and/or drilling platform or even a buoy located at a distance xe2x80x9cdxe2x80x9d beneath the surface of the water.
According to one embodiment the marine platform is used for depths of water greater than 1000 m at least.
The invention also concerns a method for sizing one or several tensioned lines Used as a means of anchoring a floating structure, the tensioned line or lines having geometric characteristics (Si and/or Di), the tensioned line or lines being made of material resistant to fatigue.
The method is characterized in that it comprises at least the following stages:
a) At least one of the natural periods T1 of heave, T2 of roll, T3 of pitch is chosen approximately within the range of periods Te of the wave excitation,
b) a value is determined for the section Si and/or the diameter Di of the tensioned line or lines,
c) depending on the external loadings to which the assembly formed by the floating structure and the said tensioned lines, force Fi is determined which acts on the tensioned line or on each of the tensioned lines,
d) the value of the stress "sgr"i, to which the tensioned line or lines are subjected, is determined,
e) the value "sgr"i is compared with an admissible maximum value "sgr"max,
f) whereas "sgr"i differs from "sgr"max, the value of section Si, and/or the value of diameter Di is varied, and stages c) to f) repeated and the value of Si and/or Di noted for "sgr"i approximately equal to "sgr"max.
According to a method of calculation starting with the value of Si and/or Di and obtained during stage f), the dynamic amplification factor FAD is determined as well as the force Fd in the said tensioned line or lines, and stages d) to f) are repeated.
According to another method of calculation, the value of heave for example is determined taking into account the value of maximum stress "sgr"max, which heave value is then compared with a tolerable value and if the heave value found exceeds the tolerable value, the value of section Si and/or the value of diameter Di of the tensioned line or lines is varied.
The method according to the invention applies for example to the sizing of tensioned lines made of high-strength composite material or of tensioned lines made of steel cables of high mechanical strength or of tensioned lines used as means of anchoring a marine platform.
The invention has the following advantages in particular:
1) the system enables the concepts currently used for production to be extended to greater depths of water, whilst keeping the costs within reasonable limits,
2) the sizing of the tensioned lines can be optimised depending on the use of the material which procures savings,
3) it reduces the influence of second-order, non-stationary phenomena associated with the vibration of the structure due to swell, known in the art as xe2x80x9cringing and springingxe2x80x9d.